Power cables employing a polymeric insulator, such as cross-linked polyethylene (XLPE), as an insulating layer have been used. However, due to space charges formed at a high direct-current (DC) electric field, paper-insulated cables having an insulating layer formed by impregnating insulating paper, which is cross-wound to cover a conductor, etc., with an insulating oil have been used as ultra-high voltage DC transmission cables.
Examples of the paper-insulated cables include an oil-filled (OF) cable in which A low-viscosity insulating oil is circulated, a mass-impregnated non-draining (MIND) cable impregnated with a high or medium viscosity insulating oil, and the like. The OF cable is limited in terms of a length of transmission of a hydraulic pressure for circulation of the insulating oil and thus is not suitable as a long-distance transmission cable. Particularly, the OF cable is difficult to install insulating-oil circulation facility at the sea bottom and thus is not suitable as a submarine cable.
Accordingly, the MIND cable is generally used as a long-distance DC transmission cable or an ultra-high voltage submarine cable.
In the MIND cable, an insulating layer is formed by winding insulating paper in a plurality of layers. For example, either Kraft paper or semi-synthetic paper formed by stacking kraft paper and thermoplastic resin such as polypropylene resin may be used as the insulating paper.
In the case of a cable in which only kraft paper is wound and impregnated with an insulating oil, when the cable is operated (when an electric current is supplied to the cable), a temperature change occurs inwardly in a radial direction, i.e., outwardly from a portion of the insulating layer adjacent to an inner semi-conductive layer, i.e., toward an outer semi-conductive layer outside the insulating layer, due to heat generated due to a joule loss due to the electric current flowing through a conductor of the cable.
Accordingly, the viscosity of the insulating oil in the portion of the insulating layer adjacent to the inner semi-conductive layer having relatively high temperature decreases and thus the insulating oil thermally expands and moves to a portion of the insulating layer adjacent to the outer semi-conductive layer. In contrast, when the temperature of the cable decreases, the viscosity of the moving insulating oil increases but does not return to the original position. Thus, deoiling voids may occur inwardly in the radial direction, i.e., in the portion of the insulating layer adjacent to the inner semi-conductive layer, due to thermal contraction the insulating oil.
In addition, when the cable is operated (when an electric current is supplied to the cable), the viscosity of the impregnated insulating oil decreases due to heat generated due to joule loss caused by the electric current flowing through the conductor of the cable and thus the insulating oil thermally expands and moves from a portion of the cable installed at a higher position to a portion of the cable installed at a lower position. When the temperature of the cable decreases, the viscosity of the moving insulating oil increases but does not return to the original position and thus deoiling voids may occur due to the thermal contraction of the insulating oil.
Because no insulating oil is contained in the deoiling voids, an electric field may be concentrated in the deoiling voids and thus partial discharge, dielectric breakdown, or the like may occur starting from the deoiling void, thereby decreasing the lifespan of the cable.
However, when the insulating layer is formed using semisynthetic paper, the insulating oil may be suppressed from flowing due to the thermal expansion of thermoplastic resin, such as polypropylene resin, which is not impregnated with oil during the operation of the cable. In addition, because an insulation resistance of polypropylene resin is higher than that of Kraft paper, a voltage shared by polypropylene may be decreased even when deoiling voids occur.
Because the insulating oil does not move in polypropylene resin, the flow of the insulating oil in a diameter direction of the cable may be suppressed due to gravity. Furthermore, surface pressure is applied to the kraft paper due to thermal expansion of the polypropylene resin at an impregnation temperature during the manufacture of the cable or at an operating temperature during the operation of the cable and thus the flow of the insulating oil may be further suppressed.
However, even if the occurrence of deoiling voids due to the flow of the insulating oil is suppressed as describe above, an insulating oil impregnated in an insulating layer, a semi-conductive layer and the like may shrink and thus a large number of deoiling voids may occur in the insulating layer and the like, when a MIND cable is installed in a low-temperature environment and used as a underground cable or a submarine cable in an extreme situation. In particular, a force may be applied to the insulating oil in the direction of gravity for a long time until electric current is supplied to the cable after the installation of the cable and thus the insulating oil may move toward the bottom of the cable. Thus, a large void is likely to occur at the top of the cable. Even when the contracting insulating oil expands again due to an increase in a temperature of the insulating layer, etc. by heat generated by a conductor during the operation of the cable, problems such as partial discharge and dielectric breakdown may be caused due to an electric field concentrated in the large void until the large void is removed.
Accordingly, there is an urgent need for a power cable, in which an insulating layer has high dielectric strength, an electric field applied to the insulating layer is effectively reduced, and particularly, a large void is suppressed from occurring in the insulating layer when the power cable is left at low temperatures for a long time until electric current is supplied thereto after installed in a low-temperature environment, thereby effectively preventing partial discharge, dielectric breakdown, etc. from occurring due to an electric field concentrated in the large void.